The present invention relates to a fixing device for heat-fixing a prefixed toner on a sheet which is the image holding body equipped in an image forming device such as copying machines, laser beam printers and the like utilizing an electrophotograph method.
The image forming device utilizing an electrophotograph method creates a manifest image by powder toner from a latent image formed on a photosensitive body having a photosensitive layer which works as a recording medium, and transfers said latent toner image onto a sheet-type paper which is the image holding body, and since said toner is yet to be fixed, heat is applied to melt the toner, and then pressure is added thereto to fix said toner image onto the paper. In order to do so, a fixing device is provided on the lower stream side of a paper conveyance passage, for example, just before a discharge portion of the paper which passes through an image forming region.
A conventional temperature control in the fixing device is disclosed for example in Japanese Laid-Open Patent Publication No. 4-295873. It is operated so as to turn the heater lamp on continuously so as to rapidly raise the surface temperature of the heat roll to the aimed set temperature Ts, in response to turning on of the power of a copying machine and the like.
Further, by detecting the reaching of a temperature To which is under the object set temperature Ts, the power supply is controlled so as to turn the heater on and off periodically, so as to prevent an overshooting caused by the rapid raise of surface temperature, reducing the ripple caused by the difference in temperature after reaching the object set temperature Ts.
Therefore, a temperature (the second temperature) To which is not aimed at performing fixing, which is lower than the set temperature (first temperature) Ts enabling fixing as the object temperature, is set in correspondence to said temperature Ts, and from the point of time where said second temperature To is detected, the control of power being supplied to the heater lamp is performed. According to such operation, the overshooting after reaching the first temperature Ts is reduced, and unnecessary power supply and the like are restrained.
According to the heat control of the fixing device disclosed in the above-mentioned publication, it is effective to control the heat roll for controlling the temperature to said first temperature Ts enabling fixing, and it is advantageous in that unnecessary power supply and the like when turning on the power could be restrained. Further, it is effective in that the ripple and the like after reaching said first temperature Ts could be minimized.
However, in the fixing device, after reaching the first temperature Ts enabling fixing, temperature control is performed constantly targeting the first temperature Ts, without changing the set temperature. Therefore, when continuous image forming is performed at the image forming portion, the heat roll and the like must be continuously rotated in order to perform a continuous fixing process at the fixing device. At this time, the heat roll is controlled and maintained to said first temperature Ts by controlling the power supplied to the heater lamp. Therefore, the lower pressing roll will be heated by the upper heat roll, and the surface temperature thereof will be raised gradually.
Therefore, the fixing performance at the fixing device when performing a continuous image forming will differ gradually between the initial image forming and during the continuous image forming. When using thin paper (for example, 60 g paper), hot offset will occur. In other words, since the surface temperature of the pressing roll is heated sufficiently and raised, the sheet of paper will also be heated from the back surface. Therefore, the toner heated by said upper heat roll is not only melted and fixed to the surface of the sheet, but also adhered to the heat roll side.
Therefore, aimed at solving the hot offset, the control temperature of the heat roll may be controlled to maintain a temperature set in advance enabling fixing but lower than the first temperature Ts. However, when setting a temperature enabling fixing which is lower than the first temperature Ts, and controlling the device to maintain this temperature constantly, a predetermined fixing performance could not be gained if the sheet of paper is thick. That is, much of the heat of the heat roll will be taken by the thick sheet, and the surface temperature of the heat roll will be reduced rapidly. Accordingly, the fixing performance will be reduced when heat-fixing the toner on the following sheets, which produces problems such as the reduction of fixing ability, fixture disorder, and reduction of brightness in the case of colored images.